The present invention relates to a numerical control apparatus for controlling machining of non-circular workpieces (hereinafter merely referred to as a workpiece) such as a cam and the like.
Conventionally, a process for grinding the workpiece such as the cam by controlling a feed of grinding wheel, the direction of which is perpendicular to a main spindle, with the numerical control apparatus in synchronism with the rotation of the main spindle is known. It is necessary to provide profile data to the numerical control apparatus for synchronizing control of the grinding wheel feed. The profile data provide the feed amount of the grinding wheel per a unit rotational angle of the main spindle so as to reciprocate the grinding wheel, that is to give it the profile creating motion along the finished shape of the workpiece.
Though the profile data are obtained from lift data of the non-circular workpiece and the grindstone diameter, the only common profile data are used from the roughing to finishing.
Since the profile data decided from the finished shape realize the finished shape faithfully in such a manner, quadratic differential components or acceleration components related to the angle are largely contained.
However, from the frequency characteristic restriction of response of the servo system, the more the large acceleration components are contained, the slower must be the machining speed for the faithful machining. That is, in order to minimize the follow-up error, the machining speed must be reduced to improve the servo system response.
When the machining speed is reduced, however, such a problem is encountered that cracks are produced in the workpiece by heating, and the slow machining speed naturally results in a poor manufacturing efficiency.
While, if the finishing profile data decided by the only finished data are used for the high-speed roughing, the feed position of the grinding wheel can not follow the command in the place where the acceleration is large, remaining the large uncut area. In this case, in the finishing performed at low-speed to improve the surface accuracy of the machining surface, since the feed amount of the grinding wheel is determined on the basis of the largest uncut area, the finishing takes much time. That is, there is no big difference on the cycle time with the case where the roughing was performed slowly from the beginning.
Since the command value is not followed when the roughing is performed at fast speed, in the worst case, over cutting is occurred and no finishing allowance is left, results in rejects.